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Boom in Pandora moths a bust for pines of far northern Arizona

FLAGSTAFF – The lab countertops at Northern Arizona University’s School of Forestry are covered with ponderosa pine branches that have several Pandora caterpillars munching away on the needles.

Richard Hofstetter, a forest entomology professor, is studying a population boom among Pandora moths in the northern reaches of the Kaibab National Forest – the most seen in the past 20 years.

It stems from a bumper crop of caterpillars last summer, before they burrowed underground and spent a year turning into moths.

“It’s important to understand our native species, and especially one that has such a high abundance and can outbreak like this,” Hofstetter said.

It’s especially important, he said, because as caterpillars the insects are prodigious eaters. In great numbers they can seriously damage a forest, as was the case last summer, when 1,000 acres of Kaibab National Forest north of the Grand Canyon wound up completely defoliated.

Shayne Rich, who with her husband owns a gas station near Jacob Lake, said that during a peak in August hundreds of moths would swarm the lights each evening.

“It was crazy. As soon as the sun went down and lights came on the moths came out. By the morning a lot of them had died,” she said. “The whole base where the cars drive was covered in moths. It took a couple hours to sweep up.”

It’s thought that the Pandora moth’s population surges around every 20 years due to a virus that controls its numbers dying off.

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When there isn’t a surge in population, Hofstetter said, over the span of a year researchers will catch 50 to 100 moths. These days their traps are catching 16,000 moths every three days.

Hofstetter said these moths have already laid eggs.

“We expect this coming summer to have a big boom of caterpillars throughout the forest,” he said.

Forest entomologist Amanda Grady said the acreage destroyed last summer was small but gives researchers an idea of what to expect from the next generation: bigger numbers and even more damage.

“That really is going to impact a large amount of acres, potentially up to 20,000 acres next year.”

Grady said the Pandora moths aren’t to blame for ponderosa pine mortality but are a contributing factor. Other stressors include bark beetles, drought and parasitic plants.

“Mistletoe is a parasitic plant that takes nutrients and water and things like that from its host,” she said. “So if trees that have a high infection rate of dwarf mistletoe they will be more likely to face mortality if combined with the defoliation.”

Grady said because the Pandora moth’s population booms are so rare it’s important to study them.

“The pandora moth is really incredible, Grady said. “We are in a really unique position right now where we have the opportunity to study an amazing event.”

Grady said some entomologists believe the moth population is controlled by a naturally occurring virus that attacks the colony sufficiently to keep down numbers for 20 to 30 years.

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“What it does to the insect’s body is it kind of liquefies it from the the inside out and dissolves its guts,” she said.

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Alexa Armstrong/Cronkite News

A case at Northern Arizona University holds Pandora moths at all stages of their two-year life cycle. Researchers have seen a population boom among the moths this year.

Pandora moth

  • Brown and green in color.
  • Wingspan of 3 1/4 to 4 1/2 inches.
  • Life cycle is two years.
  • As caterpillars, the insects eat pine needles.